Carbohydrate Metabolism Overview
Citric Acid Cycle Overview Metabolism &
Energy Summary
 Elmhurst College
Glycolysis Summary Pyruvic Acid - Crossroads  Chemistry Department
Glycolysis Reactions Glycogenesis / Glycogenolysis / Gluconeogenesis  Virtual ChemBook


Click for larger image 

Glycogenesis, Glycogenolysis,
and Gluconeogenesis

Biosynthesis of Glycogen:

The goal of glycolysis, glycogenolysis, and the citric acid cycle is to conserve energy as ATP from the catabolism of carbohydrates. If the cells have sufficient supplies of ATP, then these pathways and cycles are inhibited. Under these conditions of excess ATP, the liver will attempt to convert a variety of excess molecules into glucose and/or glycogen.

Glycogenesis:

Glycogenesis is the formation of glycogen from glucose. Glycogen is synthesized depending on the demand for glucose and ATP (energy). If both are present in relatively high amounts, then the excess of insulin promotes the glucose conversion into glycogen for storage in liver and muscle cells.

In the synthesis of glycogen, one ATP is required per glucose incorporated into the polymeric branched structure of glycogen. actually, glucose-6-phosphate is the cross-roads compound. Glucose-6-phosphate is synthesized directly from glucose or as the end product of gluconeogenesis.

Link to: Interactive Glycogenesis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.


Click for larger image 

Glycogenolysis:

In glycogenolysis, glycogen stored in the liver and muscles, is converted first to glucose-1- phosphate and then into glucose-6-phosphate. Two hormones which control glycogenolysis are a peptide, glucagon from the pancreas and epinephrine from the adrenal glands.

Glucagon is released from the pancreas in response to low blood glucose and epinephrine is released in response to a threat or stress. Both hormones act upon enzymes to stimulate glycogen phosphorylase to begin glycogenolysis and inhibit glycogen synthetase (to stop glycogenesis).

Glycogen is a highly branched polymeric structure containing glucose as the basic monomer. First individual glucose molecules are hydrolyzed from the chain, followed by the addition of a phosphate group at C-1. In the next step the phosphate is moved to the C-6 position to give glucose 6-phosphate, a cross road compound.

Glucose-6-phosphate is the first step of the glycolysis pathway if glycogen is the carbohydrate source and further energy is needed. If energy is not immediately needed, the glucose-6-phosphate is converted to glucose for distribution in the blood to various cells such as brain cells.


Click for larger image 

Biosynthesis of Glucose:

Gluconeogenesis:

Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate sources. The starting point of gluconeogenesis is pyruvic acid, although oxaloacetic acid and dihydroxyacetone phosphate also provide entry points. Lactic acid, some amino acids from protein and glycerol from fat can be converted into glucose. Gluconeogenesis is similar but not the exact reverse of glycolysis, some of the steps are the identical in reverse direction and three of them are new ones. Without going into detail, the general gluconeogenesis sequence is given in the graphic on the left.

Notice that oxaloacetic acid is synthesized from pyruvic acid in the first step. Oxaloacetic acid is also the first compound to react with acetyl CoA in the citric acid cycle. The concentration of acetyl CoA and ATP determines the fate of oxaloacetic acid. If the concentration of acetyl CoA is low and concentration of ATP is high then gluconeogenesis proceeds. Also notice that ATP is required for a biosynthesis sequence of gluconeogenesis.

Gluconeogenesis occurs mainly in the liver with a small amount also occurring in the cortex of the kidney. Very little gluconeogenesis occurs in the brain, skeletal muscles, heart muscles or other body tissue. In fact, these organs have a high demand for glucose. Therefore, gluconeogenesis is constantly occurring in the liver to maintain the glucose level in the blood to meet these demands.

Link to: Interactive Gluconeogenesis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.

Link to Rodney Boyer - Gluconeogenesis -

Quiz: How many pyruvic acid molecules are required to make glucose?  

 

 Summary of Metabolic Processes

 Metabolic Process

 Brief Description

 Starting Compound and End Product

 Energy
(Required or Given off)

 glycogenolysis
     

glycogenesis
     

 glycolysis (aerobic)
     

 gluconeogenesis
     
 

 Summary Questions:

Quiz: If you have glucose-6-phosphate, name three things you can do with it.  
If you have just eaten, have plenty of glucose in the blood, and ATP is plentiful, what happens?  
If ATP is sufficient and there are excess amino acids, what happens?